Implement some PoC of reading from disk

boot/kernel.asm

@@ -1,88 +1,78 @@
 	cpu 8086
-	org 0x7C00
+	org 0
+	jmp 0x07C0:init
 
-stack:	equ 0x7C00
+	%include "inc/bpb.asm"
-framsz: equ 0x300
 
-	jmp init
+banner:	db "rdos", 0xA, 0xD, 0
 
-	; Data on user stack while in kernel (5 words):
+	; syscalls push a register set to stack
-	; AX BP IP CS flags
+	; this is for accessing it
+rsdssi: equ 0x08
+rsesdi: equ 0x0C
+rsbp:   equ 0x10
+rslpad: equ 0x12
+rscsip: equ 0x14
+rsflag: equ 0x18
 
-	; Kernel stack:
+lpads:	times 0x30 call entry
-	; [frame] SP SS
-	;      BP-^
-	; The start of the frame is available via BP.
-	; The frame is followed by a long ptr to the user stack.
 
-	; Main entry point
+entry:	push bp
-int20h:	xor ah, ah
+	; BX+0C: ES:DI
-int21h:	push bp
+	push es
-	push ax
+	push di
-	; Test if SS=0, if it is, we assume we are already on kernel stack
+	; BX+08: DS:SI
-	mov ax, ss
+	push ds
-	test ax, ax
+	push si
-	jz rentry
+	; BX+00: AX,CX,DX,BX
-
+	push bx
-	; Save user stack
+	push dx
-	mov ax, ss
+	push cx
-	mov [cs:stack-2], ax
-	mov [cs:stack-4], sp
-
-	; Set up kernel stack
-	xor ax, ax
-	mov ss, ax
-	mov sp, (stack-4)
-
-	jmp scall
-
-	; This is like the above, except that it doesnt switch stacks.
-	; The 'user' stack data is after (below on stack) the kernel frame.
-        ; [frame] SP SS AX BP IP CS flags
-	;          \-\--^
-rentry:	mov ax, sp
-	push ss
 	push ax
 
-scall:	sub sp, framsz
+	mov ax, ss
-	mov bp, sp
+	mov es, ax
+	mov bx, sp
 
-	call getax
+	; This needs to be done when coming from userspace
-	call jmptab
+	mov ax, cs
-
-	; Restore user stack
-	mov sp, [bp+framsz]
-	mov ax, [bp+framsz+2]
 	mov ss, ax
+	xor sp, sp
 
+	int 3
+
+	; Jump back to userspace
+ujump:	cli
+	mov ax, es
+	mov ss, ax
+	mov sp, bx
+
+	; Pop whole registerset
+	; base registers
 	pop ax
+	pop cx
+	pop dx
+	pop bx
+	; string ptrs
+	pop si
+	pop ds
+	pop di
+	pop es
+	; BP is special
 	pop bp
+	; skip landing pad number
+	add sp, 2
+	; pop IP, CS and flags
 	iret
 
-	; Lookup address of subfunction
+	; Processes register set DS:BX as syscall
-	; Mesh up the stack so we return to subfunction
+scall:	; Clear status flags
-	; and subfunction later returns to scall
+	and byte [bx+rsflag], 0
-jmptab:	push ax
-	push bx
-	xor bx, bx
-	add bl, ah
-	add bl, ah
-	mov bx, [cs:bx+sftab]
-	mov [bp-4], bx
-	pop bx
 	ret
 
-	; Get AX from the user stack
+	; No such subfunction
-getax:	push ds
+sferr:	; set carry
-	push si
+	or byte [bx+rsflag], 1
-	lds si, [bp+framsz]
-	lodsw
-	pop ds
-	pop si
-	ret
-
-	; No such subfunction - set carry and exit
-sferr:	stc
 	ret
 
 	; Subfunction table
@@ -146,34 +136,93 @@ setint:	push es
 	pop es
 	ret
 
-; ===== end of resident, begin of transient startup code
+bpb:	times bpb_len db 0
+drvnum:	db 0
+	align 4
+drvpos:	dd 0
+buffer:	times 512 db 0
 
-init:	xor ax, ax
+	; Select active drive
+	; IN	dl	drive number A=0, B=1, C=2, ...
+select:	mov [drvnum], dl
+	ret
+
+	; Set sector number
+seek:	mov [drvpos], ax
+	mov [drvpos+2], dx
+	ret
+
+	; Read sector from disk
+read:	; this uses the EBIOS extensions
+
+	; qword sector number
+	xor ax, ax
+	push ax
+	push ax
+	mov ax, [drvpos+2]
+	push ax
+	mov ax, [drvpos]
+	push ax
+
+	; dword target buffer
+	mov ax, buffer
+	push cs
+	push ax
+
+	; word number of sectors
+	mov ax, 1
+	push ax
+
+	; size & passing
+	mov di, 0x10
+	push di
+
+	mov si, sp
+	mov ah, 0x42
+	mov dl, [drvnum]
+	call dnconv
+	stc
+	int 0x13
+
+	add sp, di
+
+	ret
+
+	; Convert between drive number and BIOS dl
+	; Bidirectional mapping
+	; 0 <-> 0, 1 <-> 1, 2 <-> 0x80, 3 <-> 0x81
+dnconv:	mov cx, 7
+	ror dx, 1
+	rol dl, 1
+dncl:	rcl dl, 1
+	rcr dh, 1
+	loop dncl
+	xchg dh, dl
+	ret
+
+init:	cli
+	mov ax, cs
 	mov ds, ax
 	mov es, ax
-	mov ax, 0x1000
 	mov ss, ax
 	xor sp, sp
 
-	mov al, 0x21
+	xor dh, dh
-	mov dx, int21h
+	call dnconv
-	call setint
+	call select
 
-	mov ax, 0x2520
+	xor ax, ax
-	mov dx, int20h
+	xor dx, dx
-	int 0x21
+	call seek
 
-	mov ax, 0x3520
+	call read
-	int 0x21
 
-	mov ah, 0x3f
+	mov ax, [buffer+0x1FE]
-	int 3
-	int 0x21
 	int 3
 
 main:	push cs
 	pop ds
-	mov si, msg
+	mov si, banner
 	mov ah, 0x0e
 	xor bx, bx
 loop:	lodsb
@@ -184,4 +233,3 @@ loop:	lodsb
 end:	hlt
 	jmp end
 
-msg:	db "rdos kernel stub", 0x0A, 0x0D, 0